1. Field of the Invention
The present invention relates to acoustic absorbers, and more particularly to underwater acoustic absorbers subjected to hydraulic pressure.
2. Description of the Prior Art
The present construction of underwater acoustic absorbers and reflectors subjected to hydraulic pressure consists of a rubber sheet or slab having a pattern of cavities, usually cylindrical, with the axes of the cavities perpendicular to the face of the sheet or slab. The air-filled cavities are closed on one end by on overlayer of rubber, typically 3/16 inch thick, and on the other end by rubber or the substrate. The absorber is then affixed to baffle plates by an adhesive, a time-consuming process. FIG. 1 illustrates an acoustic absorber with the cavities closed on both ends by rubber. The disadvantage of this construction is the tendency of the rubber overlayer to collapse into the cavity when the absorber or reflector is subjected to hydraulic pressure.
The collapsing of the overlayer into the cavities usually affects the acoustic impedance of the absorber or reflector in such a manner that it performs less efficiently, typically an efficiency drop from 90% to 73% at the optimum absorption frequency is observed when subjected to hydraulic pressure. Also, the collapsing of the overlayer into the cavities and the compression of the rubber body between the cavities causes the air pressure in the cavities to rise above atmospheric. The compressed air diffuses into the rubber and eventually escapes into the ambient water. A large proportion of the air in the cavities may be lost by this process if the hydraulic pressure is maintained for a long period or reapplied repeatedly. Since the air in the cavities provides part of the acoustic compliance of the absorber or reflector, its loss may produce a permanent decrease in the acoustic efficiency of the absorber or reflector.